Mostly they must be polled -- they won't talk unless asked -- so select() isn't so useful. I have a bunch of small applications driven by one central script, which is run every 5 minutes by cron.

It's really quite a mishmash of different interfaces and protocols. Each device came with its own program from the manufacturer and no integration methods(unless you bought that particular manufacturer's product solely) so we rolled our own. Some of them were quite complicated, especially the inverters which used a complicated protocol involving packets and checksums on a shared RS-485 line. I eventually hooked up a little piezo speaker to the RS-485 line itself while wrestling with the protocol, when I finally got the checksums good enough the device would actually answer, I could hear 2 clicks instead of one. Then came the trouble of dealing with the garbage and retries necessary when wrangling RS-485 without hardware timing.

Only one of them, our UPS, was simple enough to just open the serial port and read it.